Vacuum melting and pouring is used in the manufacture of many metal superalloys. Melting and pouring in a strong vacuum, such as around 1.times.10.sup.-2 Torr, not only removes harmful gases, but assures a higher purity of superalloy during the melt and in subsequent pouring. Conventionally, a large, induction heated ladle-type furnace, together with a pour mold or other holding vessel, is completely enclosed by a housing which is then evacuated. When the superalloy has been melted and refined, the ladle furnace is tilted and the melted superalloy poured directly into the mold or other holding vessel. To prevent floating slag and other impurities from entering the pour, weirs are positioned on the interior of a tundish or launder leading to the mold to capture the slag and other impurities.
However, problems have been encountered by such apparatus with the result that the quality of the poured superalloy has suffered. For example, when the furnace is tilted during a pour, often small amounts of floating slag pass beneath the weir causing low and high density inclusions to enter the melt which create unacceptable defects in the alloy. In addition, the conventional ladle-type induction furnaces use complex hydraulic and mechanical systems which, when operated in a strong vacuum, present problems in operation and maintenance. Also, a large vacuum enclosure is required for allowing clearance of the ladle-type furnace upon tilting and pouring.
A bottom-pour vacuum melt furnace would eliminate the need for a large vacuum enclosure and reduce the attendant steam injection requirements needed to pull the necessary vacuum. In addition, complex hydraulic and mechanical systems as well as slag intrusion prevention systems could be minimal. However, bottom pour systems have heretofore been found impractical for use with vacuum melting furnaces. Bottom pour systems conventionally utilize a bottom pour valve such as a sliding gate valve formed substantially from a refractory material. Often molten metal solidifies in the valve making subsequent pours impossible. An oxygen lance could remove the blockage, but its use is impractical since blown oxygen would disturb the strong vacuum balance maintained during the melt. A sand plug inserted in the valve would also prevent metal solidification. However, sand plugging is impractical since most vacuum melt furnaces require strong mixing of the melt such as accomplished through induction heating and melting. The strong mixing within a melt chamber would cause sand to dislocate from the sand plug and add impurities to the melt. To make an initial limited and small pour to remove any sand impurities would be economically impractical since the costs of superalloys are high and wastage must be eliminated.
It is therefore an object of the invention to provide a furnace for melting and pouring metal alloys having a bottom pour valve not subject to metal solidification therein.
It is another object of the invention to provide a vacuum melt induction furnace having a limited vacuum enclosure around the furnace.
It is still another object of the invention to provide a vacuum melt induction furnace having limited hydraulic and mechanical systems used for pouring metal from the furnace.